etl/docs/utilities/type-id.md

title
type_id

{{< callout type="info">}} Header: type_id.h
Since: TBC
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Below is a concise, technical overview of the etl::type_id facility, suitable for design documentation or code review context.

Overview:

etl::type_id is a lightweight, RTTI-free mechanism for uniquely identifying C++ types at runtime. It is designed for environments where standard RTTI (typeid, std::type_info) is unavailable, undesirable, or too costly—such as embedded systems or performance-constrained platforms.

The implementation guarantees uniqueness per type within a program image while remaining constexpr-friendly and requiring no dynamic allocation.

Design Concept

The core idea is to associate each distinct type T with the address of a unique static object. That address serves as the type’s identifier.

This is achieved through:

template <typename T>
struct type_id_anchor
{
  static char value;
};

Each instantiation of type_id_anchor<T> owns its own static char value, and the address of that variable is guaranteed to be unique for each distinct T. The address is stable for the lifetime of the program.

Key Components

Type Normalization

When requesting a type ID via type_id::get<T>(), the type is first normalised:

using type = typename etl::remove_cvref<T>::type;

This ensures that:

• T, const T, T&, and T&& all map to the same type_id
• Only the underlying type identity matters

Type ID Generation

template <typename T>
static ETL_CONSTEXPR type_id get() ETL_NOEXCEPT;

• Returns a type_id that uniquely represents T
• Internally stores the address of type_id_anchor<T>::value
• Requires no RTTI, no dynamic memory, and no runtime registration

Invalid Type ID

static ETL_CONSTEXPR type_id invalid_id() ETL_NOEXCEPT;

• Represents an invalid or uninitialised type ID
• Implemented as a null pointer (id == nullptr)
• Default constructor produces this state

type_id Class Semantics

Storage

const void* id;

• The identifier is stored as a pointer
• Comparison is purely pointer comparison (fast and deterministic)

Supported Operations

Operation	Behavior
Equality (==)	True if both type IDs refer to the same type
Inequality (!=)	Logical negation of equality
Ordering (<)	Pointer comparison (useful for ordered containers)
Boolean conversion	true if valid, false if invalid
Integer conversion	Explicit conversion to intptr_t / uintptr_t

Copy and Assignment

• Copyable and assignable
• Copy semantics preserve identity (pointer value)

Guarantees and Properties

• Uniqueness: Each distinct type has exactly one unique type_id
• Stability: Type IDs remain valid for the entire program lifetime
• Zero overhead: No dynamic allocation, no lookup tables
• RTTI-free: Suitable for builds with RTTI disabled
• Comparable: Can be used as keys in associative containers

Typical Use Cases

• Type-safe containers (e.g., variant, any, message buses)
• Plugin or component systems
• Compile-time / runtime type discrimination in embedded systems
• Replacement for std::type_index in restricted environments

Summary

etl::type_id provides a robust, minimal, and efficient alternative to C++ RTTI by encoding type identity as the address of a unique static object. Its design is particularly well-suited to embedded and low-level systems where determinism, performance, and binary size are critical.